Understand “current_user” concept when creating a login session in ruby

Question

I am going through the great Michael Hartl tutorial to build ruby app here.

I am trying to understand the concept of how to create a session and I am stuck in understand

Answer 1

A few things.

First, you're reading the method names wrong (which is not surprising given how cryptic ruby method naming can be). def current_user=(user) is actually read as defining the method current_user= that takes an argument user, whereas def current_user defines a method current_user that takes no arguments. These are referred to respectively as setters and getters.

Here's a reference: Ruby (programming language): What are setters and getters in Ruby?

So that explains the duplication. On to your next question.

I don't understand what does self.current_user = user means

self is a topic unto itself, worthy of its own discussion, so I won't even try to explain it (here's one reference out of many). For the purposes of this question it's just important to remember that in order to set instance variables, you need to prefix your assignment with self, even within the class (where for other purposes it would be implicit). The rest of the line is a call to the current_user= setter method I mentioned above, with the argument user.

why is it even necessary to keep this line of code - I have the cookie with the token - why do I need to know the current user?

The reason it's necessary is that you don't want to be looking up the user from the token every time you need to get the current user. Take a look at the getter method:

def current_user
  @current_user ||= User.find_by_remember_token(cookies[:remember_token])
end

What this says is: if I haven't looked up and set the instance variable @current_user yet, then look it up; if I have already set it, then just return it. That saves a lot of looking up.

I think that answers your questions. There are a lot of deeper issues (self, etc.) which you can find more information about elsewhere. Here's one discussion of why you need to include self in setters on SO: Why do Ruby setters need "self." qualification within the class?

UPDATE: Small clarification, that last link about using self for setters within the class is actually a bit off-topic, since you're calling it within a module and not directly from a class. In the context of a module, the self in self.current_user = user will become the class that the module is included inside of, e.g. User.current_user if it was called within the class User, etc. Again, another topic of discussion unto itself...